Hi missy,
The general “rule” is that the more differences there are in a mattress layering the more “differently” they will perform and “feel” but this also depends on the person on the mattress (different people sink in more or less and may feel more of different layers) and the different types of latex or other materials or components as well (Dunlop and Talalay are different for example). There are also cases that for certain people … different combinations or “pathways” of layering and construction can lead to a very similar outcome in terms of pressure relief, alignment, and even “feel”.
This is the “risk” of trying to match one mattress to another that are actually different as opposed to “matching” each mattress to a common standard (such as in post #46 here).
So the mattresses you are comparing have several differences …
First of all they have a different core (one is Dunlop in a “range” around 34 ILD and the other is Talalay? in a range (more accurate if it’s talalay) of around 36 ILD. While he ILD’s are similar … if one is Dunlop and one is Talalay then they will feel and perform differently. One is not 'better" than another … just different.
Second I’m not sure if the 26 ILD and 18 ILD layers in the first possibility you are mentioning would be Dunlop or Talalay. Rocky Mountain doesn’t have a mattress with that layering that I know of so if this is a DIY project involving separate purchases of each layer and then putting them together on your own … then any differences would depend on whether the materials are the same type of latex and whether the ILD ranges are the same as well. Any difference in material on the surface layers for many people would make even more of a difference than the core (although some people would say it was the other way around in their perception)
Finally … it would depend on whether both choices were inside the same type of cover (which will affect the performance and feel of the latex layering) or whether one or both had a topper on a finished mattress and one or both was all inside a single cover.
So comparing “mattress to mattress” takes a lot of experience, knowledge, and personal testing and it’s usually better to stick with ordering all the materials and layers from a single source so that you, with the help of the outlet, are better able to “predict” how close a mattress will come to your needs and preferences as a target rather than the performance and “feel” of another mattress that may have many differences in materials and components.
In general … there are two main approaches to choosing and buying a mattress and each has variations.
The first approach involves “becoming an expert” to different degrees. This can involve many hours, days, weeks, months, or years (some would say a lifetime) of research and experience. How much knowledge, research, and experience you would need with this approach would depend on whether you were trying to design and build your own mattress (different components from different outlets that you were putting together yourself) and how close to your “ideal” you wanted to come or you were choosing a mattress from local or online outlets that only had more limited information about their mattresses, or limited knowledge and ability to help you make your best choices, but at least had mattresses that were already put together for you to test or research. Both of these can be challenging, confusing, and frustrating although the “design and build your own” version can be fun for some as well (as long as they were willing to spend the time it takes to do the more detailed research and were comfortable with the much higher risks involved in this approach). Either of these versions of “Approach #1” are risky at best.
The second approach involves “finding the experts”. This involves some research into better local or online outlets along with just enough general knowledge and information to ask better questions, help you have a basic understanding of the information you are given, and to be able to tell when someone really knows their stuff or is just passing on the “marketing information” they use to sell mattresses. With this approach … finding the experts who sell the types of mattresses that you are most interested in and who have the choices, quality, and value that are important to you is over half the battle and the process of making good choices from outlets like this is much more enjoyable, simple, and almost always more “accurate” and closer to your long term needs and preferences. This approach will usually lead to choices between “good and good” instead of choices between “better and worse” or worse yet “unknown vs unknown”. In other words … they have the selection, quality, and value you want to include in your research and already know what you would otherwise need to learn. For most people … they are your “best friend” when you are mattress shopping (like having a mechanic with years of experience help you find a car). This is by far the least risky approach.
My suggestion in almost all cases is to avoid the “first approach” unless someone is prepared for the time, effort, research, complexity, confusion, frustration, costs, knowledge, and risk involved and the “challenge” makes all these worth it. The second approach, for the mast majority of people, is by far the better, simplest, fastest, least frustrating, least costly, least risky and most effective and enjoyable way to go.
Phoenix